Asynchronous Transfer Mode (ATM) technology is used as a backbone technology for some modern carrier networks. ATM supports network transmission of information including data as well as real-time voice and video. Networks employing ATM are typically characterized by a topology wherein network switches establish a logical circuit from one end of the network to another.
This topology functions to effectively guarantee Quality of Service (QoS) for the information transmitted over the ATM network. Inherent flexibility and efficiency typify ATM networks because unused bandwidth within the logical circuits therein can be appropriated when needed. For instance, idle bandwidth in an ATM circuit supporting a videoconference can be used to transfer bursts of data.
QoS specificity allows smooth ATM transmission of real-time critical information such as voice and video by providing a constant bit rate (CBR) to guarantee them sufficient bandwidth. Unspecified bit rate (UBR) provides a best effort for transmission of non-critical data. Applications that require minimal delay (e.g., interactive media), and bursty transaction traffic are respectively supported by real-time and non-real-time variable bit rate (rt-VBR and nrt-VBR).
ATM standards define inverse multiplexing over ATM (IMA) to aggregate multiple links (e.g., T1, E1) into a single virtual link. The virtual link provided by IMA is available for use by higher layer protocols such as the user-to-network interface (UNI), Interim Inter-Switch Signaling Protocol (IISP), ATM Inter-Network Interface (AINI) and private network-to-network interface (PNNI). Virtual links such as label switched paths (LSP) can also be provided by multi-protocol label switching (MPLS). ATM Virtual Path Connections (VPCs) can also comprise a virtual link.
Prior Art FIG. 1 depicts an exemplary conventional ATM network 10. Within network 10, a virtual link 12 between nodes 1 and 2 comprises the aggregation (e.g., IMA) of five T1 links 1T1-5T1. By ATM standards, virtual link 12 comprises the aggregation of the T1 links. Thus from the perspective of the control plane 11, virtual link 12 is treated as a single, individual link, rather than as five independent links 1T1-5T1. Nodes 2 and 3 are linked by link 13.
For an IMA, group usage can be defined as two components. The maximum bandwidth of an IMA is the sum of the bandwidths of the individual link components comprising it. For example, the maximum bandwidth of IMA 12 is the sum of the individual bandwidths of 1T1-5T1. The Minimum link bandwidth is the minimum bandwidth guaranteed for the virtual link corresponding to the IMA. Minimum bandwidth is guaranteed aside from that of individual link components and is the bandwidth needed for the virtual link (e.g., IMA) to be operational, specified in number of links. It is appreciated that these definitions apply in the IMA plane
Minimum link bandwidth is guaranteed notwithstanding any failure of the individual links comprising the virtual link. For instance, the minimum bandwidth of IMA 12 is guaranteed even if some of links 1T1-5T1 fail. From a perspective of the network control plane, group badwidth of a virtual link is binary; the link is either up (e.g., available) or down (e.g., unavailable). However, this conventional definition provides no informative granularity as to actual available bandwidth.
Without such granularity, upper layer protocols such as PNNI and MPLS are unaware of the actual available bandwidth of their constituent virtual links. The minimum link bandwidth of exemplary virtual link 12 is achievable as long as a minimum of two of the individual links 1T1-5T1 are available. Thus if links 3T1-5T1 fail, IMA 12 is still up, from a conventional perspective, as long as links 1T1 and 1T2 remain available.
However, the actual maximum bandwidth now available at IMA 12 has effectively degraded 60% to the sum of the bandwidths of only two individual T1 circuits, 1T1 and 2T1. Conventionally, PNNI still treats virtual link 12 as having 250% more bandwidth than is actually now available. The virtual link may thus be unable to effectively handle traffic that exceeds 40% of its defined maximum bandwidth. This can result in congestion, dropped data and QoS degradation.
For instance, where IMAs are deployed in locales wherein the connection costs of T1 or E1 (T1/E1) are high, they are typically loaded with connections that consume close to the maximum bandwidth. PNNI IMA links are becoming more commonly deployed, such as to route voice calls over IMA links. Congestion, dropped data and QoS degradation are inconsonant to voice calls and other such CBR services; they tend to degrade call quality, even significantly.
Where an IMA group (or e.g., a MPLS LSP, link bundle, VPCs, etc.) functions as a PNNI trunk, link PNNI topology state elements (PTSE) advertise the maximum bandwidth of the IMA group. Connections are routed through the link according to this maximum advertised bandwidth. A PNNI may sometimes route connections onto the IMA group so as to consume close to 100% of the maximum bandwidth of the link.
Where a failure occurs in one or more of the T1/E1s (or e.g., LSPs, individual links, VPCs, etc.) in the IMA group (MPLS, link bundle, etc.), the group bandwidth drops. The link stays up if its actual available group bandwidth remains at or above its minimum guaranteed bandwidth. The IMA group is up as long as the group satisfies the condition of the minimum number of retained T1/E1 links. Where connections consume close to 100% of the maximum bandwidth, the data plane can become congested.
There may be within the network underutilized paths available, but as the established connections are through the IMA group (or e.g., LSP bundle, link bundle, VPC bundle, etc.), the congestion goes unabated. Congestion and data loss in the data plane are not reflected in the PNNI control plane because the degradation in actual available bandwidth is not communicated to the control plane. The PNNI thus takes no responsive action, such as release of the connections (e.g., of the bandwidth-degraded virtual link).
Under these conditions, the node conventionally continues trying to send all configured traffic over the bandwidth-degraded IMA group. In the face of insufficient available bandwidth congestion, data is randomly discarded. Data connections (UBR) so oversubscribed may adjust to the available bandwidth, such as by Transfer Control Protocol (TCP) windowing and other techniques. Voice calls (and/or other CBR e.g., video) however undergo statistically even degradation.
Under conditions of severe congestion on the data plane resulting from marked degradation in actual available bandwidth over a constituent virtual link, problematic results may become apparent. For example, those skilled in the art are aware of the so-called “last straw” problem, wherein a new voice call over a congested network degrades the quality of all of the calls then in progress over that network. This can be very problematic for voice and data calls and the like.
Calls and other network traffic can be re-routed by the control plane as quality problems become apparent. However, calls have already sustained degraded quality by this point. Such reactive re-routing can also impact available bandwidth in the newly configured links. Further, fluctuations in the available bandwidth through any of the network's links can diminish network stability by causing flapping to occur, in which the availability of a route toggles.
Conventionally, configuring the PNNI bandwidth to be the minimum number of IMA links implicitly grossly undersubscribes the link. Configuring the PNNI bandwidth to be the maximum number of IMA links however unwittingly oversubscribes the link in the event of the failure of an individual component line of that link, since the PNNI control plane is unaware of the degradation in actual available bandwidth and thus fails to release calls in an organized fashion.
The issues described above are not unique to IMA groups, but are rather characteristic of PNNI trunks tunneled using other technologies as well. For instance, the issues apply to PNNI trunks tunneled using multiple MPLS LSPs. To provide availability, multiple MPLS transport LSPs are used as a single PNNI trunk. Failure of individual component LSPs thus degrade the actual available bandwidth of the MPLS configured PNNI trunk, as discussed above (e.g., IMA configurations). These issues also apply to other aggregated links, such as ATM VPCs, etc.
The issues also arise with other control planes that signal connections which traverse virtual interfaces configured as bundles. These virtual interfaces include Multilink Point-to-Point Protocol (MLPPP), Multilink Frame Relay (MFR), and other bundles in which the control plane routing and signaling is not aware of the bundling.
Conventional approaches can thus be problematic. Failure of individual component links comprising PNNI trunks tunneled using IMA groups, multiple MPLS LSPs, ATM VPCs, and/or other technologies reduce the actual available bandwidth of the PNNI trunk, yet without control plane awareness thereof. Control plane action responsive to the degradation in actual available bandwidth on the data plane is thus not triggered. Congestion, data loss, flapping, and/or QoS degradation can result.